A high-pressure discharge lamp tends to suffer from unstable discharge immediately after it is triggered or when it comes close to the end of its lifetime. It is well known that the discharge lamp may blink, or fade out in the worst case even if a lighting device supplies power.
As shown in FIG. 21 of the accompanying drawing figures, rectification discharge is known as one of causes of the foregoing problems. With the rectification discharge, emission of electrons between a pair of electrodes of an AC-activated discharge lamp becomes asymmetric in anodic and cathodic cycles. This is caused by unreliable formation of so-called luminescent spots, i.e. unstable discharge of thermal electrons, since one electrode in the cathodic cycle cannot shift to arc discharge from glow discharge. Such a phenomenon is somewhat inevitable to the discharge lamp. In the case shown in FIG. 21, it is known that an electrode 1 cannot sufficiently discharge electrons in the cathodic cycle.
It is conceivable that the unstable state of the luminescent spots of the electrodes are triggered because the electrodes or radioactive substances in electrons are exhausted, or because discharging functions of the discharge lamp become unstable due to impurities.
In any case, the discharge lamp suffers from increased impedance, and blinks or fades out due to insufficient power of the lighting device if no countermeasures are taken.
Further, there is a phenomenon in which the discharge lamp is turned on once, and transiently and abruptly increases its impedance. Especially, this phenomenon is remarkable with a so-called metal halide lamp in which a metal halogen compound is sealed in a discharge tube of the discharge lamp as a light-emitting substance. Specifically, the phenomenon is observed when the discharge lamp is in a startup mode and when it is in a steadily lighting mode.
The foregoing phenomena will be described with respect to behavior of the metal halide lamp. In the startup mode, the lamp is triggered by simultaneous dielectric breakdown of the light emitting substances and a combination of inert gases and mercury vapors, both of which are sealed in a discharge tube. In this state, the metal halogen compound has a low vapor pressure, and hardly contributes to discharge.
Thereafter, electric power is applied to the discharge tube for several minutes. When the discharge tube becomes hot, the light emitting substance becomes loose, the vapor pressure of the metal is raised, and a lamp voltage is increased.
What is unexpected in the foregoing process is that the vapor pressure of the metal is raised instantly and abruptly if the light emitting substance is present at an unstable spot, e.g. near a hot electrode, and comes into contact with the hot electrode. In such a case, if a magnetic ballast whose power supply performance depends upon the commercial power supply, a voltage of the lamp exceeds an output of the ballast, so that the lamp fades out. A similar phenomenon is observable when the commercial power supply is interrupted in a split second and when the commercial power supply voltage is instantly decreased.
On the other hand, an electronic ballast includes a so-called inverter circuit which lights the lamp with the rectangular wave AC, and controls lamp power to be approximately constant. If the lamp voltage varies as stated above, a lamp current is reduced. As a result, the lamp impedance is transiently raised, which makes the lamp fade out. Referring to FIG. 22, the lamp voltage is abruptly raised from a value at an operation point 1 to a value at an operation point 2.
Further, there is another reason for the increase of the lamp voltage. For instance, even a good lamp may suffer from an increased voltage with a lapse of time. This is caused by chemical reactions of the substance sealed in the lamp, or release of impurities. This phenomenon is essentially inevitable, and leads to the foregoing phenomena.
The following describe behavior of the ballast. With the ballast used for an AC-powered discharge lamp, a lamp current is turned off once each time polarity reversal is performed in a half cycle. In order to restart the lamp in a next half cycle, the lamp voltage from the ballast is supplied first while the lamp current which is emission of thermal electrons of the electrodes is supplied after a while. Therefore, impedance is transiently increased as shown by a white circle. The white circle denotes a transient value of the high impedance in the half cycle.
When the lamp is activated by the commercial power having sine waves with delayed rising edges, the lamp voltage becomes a so-called re-striking voltage which is abruptly raised after the zero-cross. This makes the lamp fade out when the power supply voltage becomes insufficient.
On the other hand, with a rectangular wave inverter circuit whose waveform quickly rises, an output voltage can be advantageously controlled to be constant. However, when the lamp voltage is also controlled to be approximately constant, the lamp current is decreased in spite of the increase of the lamp voltage. Referring to FIG. 24, the lamp impedance is transiently raised, which causes the lamp to fade out. In FIG. 24, the lamp is operated at an operation point 2′ where the lamp voltage is increased when an ideal power source (a constant voltage source, for example) is used. However, the impedance is raised to the value at the operation point 2 along an output line of the constant power supply.
Japanese Patent Laid-Open Publication No. Sho 60-250599 discloses a discharge lamp lighting device, which includes a DC-DC converter having current limiting characteristics, and a rectangular wave inverter converting an output of the DC-DC converter into a rectangular wave AC. The discharge lamp lighting device supplies the output of the rectangular wave inverter to the discharge lamp via a high voltage pulse superimposing circuit. The discharge lamp lighting device controls current limiting characteristics in response to detected output values of the DC-DC converter, output current and discharge tube voltage. However, the lighting device does not control the current flowing to switching elements to a predetermined peak value in each switching cycle when the discharge lamp tends to fade out.
In summary, the phenomena which cause unstable lighting and fading out of the discharge lamp are not favorable to the discharge lamp and the ballast for the following reasons.
(1) The lamp asymmetrically discharges.
(2) The lamp increases the impedance due to the transient behavior or aging.
(3) The ballast does not have a sufficient output (or power supply performance).
An electric model of the discharge lamp will be reviewed hereinafter.
FIG. 25 is a graph showing voltage-current characteristics of the discharge lamp from its startup mode till its steadily lighting mode. For convenience sake, the voltage-current characteristics per half cycle are depicted.
At an operation point (a), the discharge lamp undergoes voltage breakdown by high voltage pulses coming from outside. In this state, the discharge lamp remains in a transient state between the glow discharge and the arc discharge, and has high impedance.
When an appropriate ballast output is supplied, the lamp changes to a mode shown at an operation point (b) where the lamp is quasi-stable. In this state, the discharge lamp has undergone current breakdown, but maintains a high lamp voltage, and has impedance which is not lowered completely.
Next, when a ballast output which can apply an approximately rated current, the lamp completely changes its state to a state shown at to an operation point (c) where the impedance and a voltage are low, and a lamp current is large.
Thereafter, the lamp voltage gradually increases as shown by an output curve of the ballast. The lamp becomes stable at a rated operation point (d) along with an increase of the impedance.
When the lamp is new, it becomes stable at the operation point (d) each time it is lit. With a lapse of time, the lamp gradually increases its voltage as shown at operation points (d′)→(d″)→(d′″).
In FIG. 26, the lamp voltage is in a normal range at the operation points (d′) to (d″) while it is abnormal at the operation point (d′″).
The lamp impedance is variable as shown by dashed lines.
Further, the lamp is assumed to be in operation with a constant current between the operation points (c) and (d′″).
In order to overcome the foregoing problems, it is necessary to apply optimum ballast outputs at the respective operation points (a)→(b)→(c)→(d)→(d′)→(d″)→(d′″).
The present invention has been contemplated in order to overcome problems of the related art, and is intended to provide a high-pressure discharge lamp lighting device which can protect a discharge lamp against unstable lighting and fading out from the startup till the end of life.